1. Field of the Invention
The invention relates generally to pressure relief devices for large, power distribution equipment such as transformers and switches of the type that are filled with a dielectric oil or the like and which, when a fault occurs, are subject to damage caused by increased pressure.
2. Description of Related Art
Pressure relief devices, commonly referred to as PRDs, must quickly relieve the increased pressure that can accumulate in housings containing such electrical distribution equipment by allowing large quantities of potentially very hot fluids to escape from the housings in a short time. The devices must respond before the housings in which the equipment is contained rupture and in so responding allow a sufficient amount of fluid to be discharged in a sufficiently short amount of time to prevent damage, all while, preferably, discharging the fluid in a manner where it will not cause injury or damage to persons or equipment located close by.
Typically, pressure relief devices of type to which this invention is addressed have a two-stage operating characteristic. When the pressure in the housing increases to a predetermined threshold level requiring relief, a first valve opens. The release of pressure through the first valve opens a second interconnected valve, which has the effect of further opening the first valve to discharge fluid from the housing at a higher rate. The second valve, which operates once the first valve is opened, itself opens at a lower pressure than required to open the first valve. The lower pressure operating characteristic of the second valve together with its physical association with the first valve enables the second valve to open the first valve wider and to maintain the first valve open for a longer period of time until the pressure in the housing is well below the threshold required for opening the first valve.
One way in which this is accomplished is to provide two different size valve openings, a first-stage internal valve opening associated with a first seal circumscribing a first area and a second-stage external valve opening associated with a second seal circumscribing a second larger area. The first-stage internal valve opening is formed by an exit opening through a tubular collar aligned with a vent opening in the housing. The second-stage external valve opening is formed on a peripheral surface of the tubular collar. A preloaded poppet in the form of a disk engages both seals. An endface of the valve disk engages the first seal with a preload force produced by a compression spring. A depending skirt of the valve disk engages the second seal with a sliding fit. When the product of the pressure in the housing times the first area exceeds the preload force exerted by the compression spring on the valve disk, the valve disk is displaced for opening the first valve. Once the first valve is opened, an enlarged area of the valve disk corresponding to the second area is exposed to the pressure in the housing. The force generated by the product of the housing pressure times the second area is larger than the product of the housing pressure times the first area by a ratio of the two areas, which is typically around 2 to 1. The larger force further displaces the valve disk for opening the second valve and for opening the first valve even wider. Both valves are opened widely and quickly because the force generated by the application of housing pressure over the larger second area of the second valve greatly exceeds (such as by a factor of two) the preload force imparted by the compression spring at the closed positions of the valves.
PRDs are mounted onto the electrical housings using mounting flanges that are adapted by convention to fit a pattern of bolts surrounding the vent openings in the housings. The arrangement of such bolts is more or less standardized in the industry and imposes a limitation on the configuration of the PRD, which has, as will be described below, restricted the rate at which fluid may flow through the PRD.
The mounting flange has an annular working area that provides clearance for bolting the mounting flange to the housing. The working area is defined between effective inner and outer diameters that ensure a minimum clearance for bolting the mounting flange to the housing. The tubular collar, which has an inner diameter set by the first-stage internal valve opening and an outer diameter set by the second-stage external valve opening, projects from the flange within the inner diameter of the working area. Thus, the inner diameter of the working area of the flange determines the maximum size of the outer diameter of the tubular collar and the second-stage external valve opening. The first-stage internal valve opening is required to be proportionally smaller in diameter to provide the difference between the first and second areas required for the desired two-stage operation. It is the size of the first area that determines the maximum discharge rate through the PRD, and the limitations imposed upon this first area by the combination of (a) the inner diameter of the working area of the flange and (b) the further diametrical reduction required for two-stage operation significantly restricts the maximum flow rate through the PRD.
An example of a PRD representative of current designs is shown in FIG. 1. The illustrated PRD 10 is mounted over a vent opening 14 in an electrical equipment housing 12 with a flange 16 that is secured to the housing 12 by a circle of bolts 18. The flange 16 has an annular working area 20 defined between inner and outer diameters 22 and 24 occupied by the bolts 18 and any washers or other fastening elements required to attach the flange 16 to the housing 12. A tubular collar 30 formed integrally with the flange 16 has a first-stage internal valve opening 32 near an innermost diameter 34 of the tubular collar 30 and a larger second-stage external valve opening 36 on an outermost diameter 38 of the tubular collar 30. The outermost diameter 38 of the tubular collar 30, which includes the second-stage external valve opening 36, is located within (i.e., is smaller than) the inner diameter 22 of the working area 20 of the flange 16 for preserving the clearance required for mounting the PRD 10 on the housing 12 using the conventional bolt pattern. The innermost diameter 34 of the tubular collar 30 is constricted by the first-stage internal valve opening 32 that is necessarily proportionally smaller than the second-stage external valve opening 36 to support the desired two-stage operation. Flow rates through the PRD 10 are limited by the first-stage internal valve opening 32 that constricts the innermost diameter 34 of the tubular collar 30.
Within the tubular collar 30 is a throat 40 having an entrance adjacent to the vent opening 14 in the housing 12 and an exit adjacent to the first-stage internal valve opening 32. At its entrance, the throat diameter 42 is as least as large as the diameter 44 of the vent opening 14 and is limited only by the inner diameter 22 of the working area 20 of the flange 16 and the thickness of the tubular collar 30. At its exit, the throat diameter 46, which corresponds to the innermost diameter 34 of the tubular collar 30, is significantly smaller than the diameter 44 of the vent opening 14 in the housing 12 and is limited by the size of the first-stage internal valve opening 32. Potential flow rates through the tubular collar 30 are reduced by the difference between the entrance and exit diameters 42 and 46 of the throat 40 or at least by the difference between the diameter 44 of the vent opening 14 in the housing and the limited exit diameter 46 of the throat 40 (i.e., the innermost diameter of the tubular collar 30).
Although conventional PRDs have, for many years, provided effective pressure relief operations for electrical equipment housings, it can be readily be appreciated by reference to FIG. 1 that the size of the first-stage internal valve opening 32 imposes a limit on the rate at which fluid may flow through the pressure relief device and that this limit is lower than the limit imposed by the size of the vent opening 14 in the housing 12. Accordingly, to achieve higher flow rates commensurate with the size of the vent openings in electrical equipment housings, larger size PRDs have been required, which involve expensive changes to the electrical housings to accept pressure relief devices having larger footprints (i.e., flange diameters). High costs generally make such retrofits prohibitively expensive.